1. Field of the Invention
The invention relates to a solids reducing downcomer. More particularly, the invention relates to a downcomer for producing a gas and solids reduced slurry from a three phase hydrocarbon synthesis slurry comprising gas bubbles and particulate catalyst solids dispersed in a hydrocarbon liquid.
2. Background of the Invention
Slurry hydrocarbon synthesis (HCS) processes are known. In a slurry HCS process a synthesis gas (syngas) comprising a mixture of H.sub.2 and CO is bubbled up as a third phase through a slurry in a reactor in which the slurry liquid comprises hydrocarbon products of the synthesis reaction and the dispersed, suspended solids comprise a suitable Fischer-Tropsch type hydrocarbon synthesis catalyst. Reactors which contain such a three phase slurry are sometimes referred to as "bubble columns", as is disclosed in U.S. Pat. No. 5,348,982. Irrespective of whether the slurry reactor is operated as a dispersed or slumped bed, the mixing conditions in the slurry will typically be somewhere between the two theoretical conditions of plug flow and back mixed. The catalyst particles are typically kept dispersed and suspended in the liquid by the lifting action of the syngas bubbling up through the slurry and by hydraulic means. Mechanical means such as impellers and propellers and the like are not used, because they will quickly erode and also cause attrition of the catalyst particles. One or more vertical, gas disengaging downcomers may be used as hydraulic means to assist in maintaining more uniform catalyst dispersion, by providing a vertical catalyst circulation in the slurry, as is disclosed in U.S. Pat. No. 5,382,748. The catalyst particles, being heavier than the liquid, tend be more concentrated at the bottom of the slurry. It would therefore be advantageous in maintaining a more uniform vertical catalyst concentration to circulate a catalyst lean slurry to the bottom. Further, the slurry liquid hydrocarbon product of the HCS reaction must be separated from the catalyst particles. This is typically accomplished by mechanical filtration in which the slurry is fed to one or more porous filter media which permit the liquid to pass through, but not the catalyst particles. The hydrocarbon liquid filtrate is then sent to further processing and upgrading. The build-up of a catalyst particle cake and plugging of the filters could be reduced if some of the catalyst particles were removed from the slurry before it is filtered. Hence, there is a need for a means of reducing the catalyst concentration in slurry being fed to the bottom of the reactor and to filtration.